i too had to pull the bullets from several hundred rounds of 5.56mm the powder charge was too low, thus causing many failures to cycle. if the cases were crimped i recommend the following.., i have a Dillon XL650, i removed the primer advance part and the de-capping pin then proceeded to reload the already primed cases, the cases went thru the sizing die with ease, thus reforming the neck.

Actually, having had a bullet seated will do more for uniform neck tension than re-sizing. After seating a bullet, the neck ID should be at the perfect spot--0.001" under bullet OD. Measure some and see. This has been my finding for many cartridges--the expander gets me 0.002-0.003" under, where "perfect" is 0.001" and is found by getting a custom expander ball or, as in this, previously seating a bullet.
Your only concern is old "thumb pressure on the case with bullet nose on bench" set-back test, that you should be doing all the time any way.
Sometimes you already HAVE consistency and anything you do will simply lessen the consistency you want.

Quote: Its the firing of the round that expands the brass, not seating the bullet....

Well let me tell you a story...........

About a year ago I decided that the 150 grn Swift Sirroccos were not going to group like I'd hoped for,,, so I pulled 15 bullets and poured the powder back in the can ans set the cases on the shelf until one day I came up with a new bullet to try in my 7mm rem mag, the 140 grn Accubond, as it turns out there IS a slightly difference in these two bullets,,, cause after I loaded a few they just would not pass the Thumb Pressure on the ole bench Test, after posting my said troubles on this forum another member pointed out to me that I could take my expander ball and primer pin out of my die and resize the casemouth, and it worked to perfection of course.( thanks Taylorce1)

I would recommend in this case, you measure the OD of the bullets and the ID of the case neck and determine where the problem is.
It isn't that uncommon to find that even jacketed bullets will be a bit smaller than the label says (I would say they could be larger, but I have never seen it).
I have gotten 0.354" 9mm bullets from the "big guys", when the box was clearly labeled 0.355".
Caliper and micrometer will find the cause of many problems.

Seating bullets does; however slightly, expand the case neck. If it didn't expand the neck the brass wouldn't spring back to hold the bullet secure. You needn't full length resize, just back the sizing out until only the case neck is resized. This way your case will be the same size as when you started and future loadings wii be identical to your first.

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